1. Field of the Invention
This invention relates generally to endoprostheses and, more particularly, to a system and method for coating endoprostheses such as stents.
2. Description of the State of the Art
The present invention involves radially expandable endoluminal prostheses or endoprostheses, which are medical devices adapted to be implanted in an anatomical lumen. An “anatomical lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of these endoprostheses. Stents are generally cylindrically shaped devices, which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels.
Stents have been made of many materials including metals and polymers. Polymeric materials include both nonbioerodable and bioerodable plastic materials. The cylindrical structure of stents is typically composed of a scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be made of a base material that is metallic or polymeric and can be in the form of wires, bars, tubes, or planar films of material rolled into a cylindrical shape.
FIG. 1 shows an endoprosthesis 10 in the form of a stent. The endoprosthesis 10 includes a plurality of ring struts 12 that define a plurality of undulating hoops or rings 14 having an initial diameter. The endoprosthesis 10 can include any number of rings 14 to achieve a desired overall length. The rings 14 are capable of being compressed in a radial direction 16 to a diameter smaller than the initial diameter and are connected to each other by a plurality of connecting struts 18. The rings may also include any number of ring struts 12 to achieve a desired diameter. The endoprosthesis 10 is in the shape of a hollow cylinder or tube (illustrated in broken lines) having a central axis 20. Each ring strut 12 and connecting strut 18 has an outer or abluminal surface 22 that faces away from the central axis 20 and an inner or luminal surface 24 that faces toward the central axis.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents or drug eluting stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or even toxic side effects for the patient. Local administration is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local administration thus produces fewer side effects and achieves more favorable results.
A medicated endoprosthesis may be fabricated by coating the surface of either a metallic or polymeric scaffolding to produce a drug reservoir layer on the surface. The drug reservoir layer typically includes a polymeric carrier that includes an active agent or drug. To fabricate a coating, a polymer, or a blend of polymers, can be applied on the endoprosthesis using commonly used techniques known to those having ordinary skill in the art. A composition or substance to be coated onto the endoprosthesis may include a solvent, a polymer dissolved in the solvent, and an active agent dispersed in the blend. Conventionally, the substance is applied to the endoprosthesis by immersing the endoprosthesis in the substance or by spraying the substance onto the endoprosthesis.
As used here, “endoprosthesis” encompasses devices that are implanted permanently and temporarily inside a human or animal body. Metallic stents are permanently implanted, whereas polymeric stents are usually designed to resorb into the body. Temporarily implanted or non-permanent endoprostheses are removed from the patient after a relatively short indwelling time. Temporarily implanted endoprostheses can be a catheter system, such as a balloon catheter used dilate a constriction within body lumen during angioplasty and a stent delivery system used to position and deploy a stent with a body lumen. Temporarily implanted endoprostheses can be medicated to provide localized delivery of therapeutic or beneficial agents. An outer surface coating can be used as a vehicle to carry the agents.
An endoprosthesis coating should meet several well-known criteria including mechanical integrity, controlled release of the drug, and biocompatibility. A primer layer can serve as a functionally useful intermediary layer between the surface of the device base material and an active agent-containing or reservoir coating, or between multiple layers of reservoir coatings. The primer layer provides an adhesive tie between the reservoir coating and the device base material. Also, a barrier or polymeric topcoat layer above a reservoir layer serves the purpose of controlling the rate of release of an active agent or drug. An endoprosthesis may also include a biobeneficial coating over a reservoir layer and/or topcoat layer to improve the biocompatibility of the coating.
It would be advantageous to reduce the amount of coating material on certain surfaces of an endoprosthesis without adversely impacting the treatment capabilities. For instance, the presence of a topcoat layer, such as a poly(ester amide) (PEA) layer, on a luminal surface can have a detrimental impact on the implantation process and coating mechanical integrity. PEA topcoats can change the coefficient of friction between an endoprosthesis and a delivery device carrying the endoprosthesis to the implantation site. In addition, some PEA polymers have structures that cause them to be sticky or tacky, thereby inhibiting the smooth release of an endoprosthesis from a delivery device. Therefore, it would be desirable to limit the presence of PEA topcoats on the luminal surface.
It may also be advantageous to have an endoprosthesis with an asymmetric coating in which a different therapeutic agent is applied to the abluminal than the luminal surface. For example, an angiogenic substance and/or an anti-thrombotic substance can be applied to the luminal surface which faces flowing blood when the endoprosthesis is implanted in a blood vessel. The angiogenic substances can stimulate or promote formation of one or more collateral vessels downstream of, or distal to, the site of implantation. The collateral vessels facilitate restoration of blood flow to deprived tissue, thereby constituting “natural bypasses” around an occluded vessel. The anti-thrombotic substance can inhibit clogging of the endoprosthesis as a result of interaction between the endoprosthesis and blood flow therethrough. In order to treat restenosis, a substance that inhibits abnormal migration and/or proliferation of smooth muscle cells can be applied to the abluminal surface which would face the blood vessel walls after implantation.
Conventional methods of loading an endoprosthesis with beneficial agents by either dipping or spraying often require coating the entire endoprosthesis or large portions of the endoprosthesis with the beneficial agent. Additionally, if it is desired to superimpose two or more conventionally-loaded prostheses, such as with nested stents or bifurcated stents, the total dosage of beneficial agent to the lumen may well exceed the nominal or optimal dosage. Another drawback of conventional coating methods is the lack of or difficulty in achieving selective dosing that provides various beneficial agents or various concentrations of the same beneficial agent at different locations on an endoprosthesis.
The features and advantages of the invention will be more readily understood from the following detailed description which should be read in conjunction with the accompanying drawings.